This invention relates to recovery of oil from a subterranean reservoir through the use of a surfactant-cosurfactant system.
It has long been known that the primary recovery of oil from a subterranean formation leaves a substantial amount of the oil still in the formation. This has led to the use of what is commonly referred to as secondary recovery or water flooding wherein a fluid such as brine is injected into a well to force the oil from the pores of the reservoir toward a recovery well. However, this technique also leaves substantial amounts of oil in the reservoir, so-called residual oil, because of the capillary retention of the oil. Accordingly, post-primary surfactant systems have been employed either in place of the secondary recovery or more generally in a tertiary recovery process. One particularly suitable type of surfactant system is that which results in the in situ formation of a microemulsion which is immiscible with the oil it is displacing. Such microemulsion systems are very effective in removing residual oil. However, these systems can suffer from a deterioration of the surfactant system as it moves through the formation due to alteration of the cosurfactant concentration.
The surfactant systems employed to produce microemulsion type oil recovery basically contain at least three separate ingredients, brine, a surfactant and a cosurfactant. It is disclosed in Glinsmann, U.S. Pat. No. 4,125,156, issued Nov. 14, 1978, how to systematically optimize a system so as to give a combination of surfactant, cosurfactant and brine which produce low interfacial tension which is associated with good oil recovery.